The present invention relates to a process for preparing a corrosion-resistant trivalent chromium coating on a metal, preferably aluminum and aluminum alloys, and an improved acidic aqueous solution for use in the process.
Conversion coatings have been widely used in metal surface treatment for improved corrosion inhibition and improved adhesion of a subsequently applied paint layer. Conversion coatings are applied through chemical reactions between the metal and the bath solution which converts or modifies the metal surface into a thin film with required functional properties. Conversion coatings are particularly useful in surface treatment of metals such a steel, zinc, aluminum and magnesium. In the past, chromate conversion coatings have proven to be the most successful conversion coatings for aluminum and magnesium. However, chromate conversion coatings used in the past generally contained highly toxic hexavalent chromium. The use of hexavalent chromium results in potential hazardous working conditions for process operators and very high costs for waste disposal.
In order to overcome the problems associated with hexavalent chromium containing conversion coatings, there has been an effort to employ trivalent chromium conversion coatings which are far more acceptable from an environmental standpoint. U.S. Pat. Nos. 4,171,231, 5,304,257 and 5,374,347 disclose trivalent chromium solutions for use in forming conversion coatings on metals. One drawback of these trivalent chromium processes and acidic aqueous solutions is the formation of chromium containing precipitate in the processing bath solution over time. The precipitation results in material loss in the solution and affects coating quality when the concentrations of key components drop below desired and required levels.
Accordingly, it is the principal object of the present invention to provide a more stable solution by adding complexing agents that can form soluble complexes with trivalent chromium in solution and thereby prevent the solution from precipitation.
In accordance with the present invention the foregoing object is readily obtained.
In accordance with the present invention, an acidic aqueous solution containing a water soluble trivalent chromium compound is provided with a solution stability additive for reducing precipitation of trivalent chromium over time. The solution stability additive comprises a complexing agent selected from the group consisting of organic acids (single coordination acids and bidentate chelating compounds) and amino acids. The concentration of the solution stability additive varies based on the complexing capability of the additive. Suitable additives for use as solution stability additives in accordance with the present invention are selected from the group consisting of acetic acid, glycolic acid, and mixtures thereof.
For the features of the present invention will be made clear from the following detailed description.
The present invention relates to a process for preparing a corrosion-resistant trivalent chromium coating on a metal, preferably aluminum and aluminum alloys, and an improved acidic aqueous solution for use in the process.
The process for preparing a corrosion-resistant trivalent chromium coating on aluminum and aluminum alloy substrates comprises treating the substrates with an acidic aqueous solution, which is free of hexavalent chromium, comprising a water soluble trivalent chromium compound, a water soluble fluoride compound, an alkaline pH adjustment reagent and a solution stability additive for reducing the precipitation of trivalent chromium. In accordance with the present invention, the solution stability additive comprises a complexing agent selected from the group consisting of organic acids and amino acids. Generally, the solution stability additive is present in an amount of between 1xc3x9710xe2x88x924 moles/liter (M/l) and 1xc3x9710xe2x88x922 M/l with respect to the total acid solution, preferably between 1xc3x9710xe2x88x923 M/l and 8xc3x9710xe2x88x923 M/l with respect to the total acidic aqueous solution. Particularly suitable additives for use as a solution stability additive are selected from the group consisting of acetic acid, glycolic acid, and mixtures thereof.
The acidic aqueous solution to which the solution stability additive is introduced comprises a water soluble trivalent chromium compound, a water soluble fluoride compound and an alkaline reagent. The trivalent chromium compound is present in the solution in an amount of between 0.2 g/liter to 5 g/liter (preferably between 0.5 g/liter to 2 g/liter), the fluoride compound is present in an amount of between 0.2 g/liter to 5 g/liter (preferably 0.5 g/liter to 2 g/liter), and the alkaline reagent is present in an amount to maintain the pH of the solution between 3.0 to 5.0 (preferably 3.5 to 4.0).
By providing the solutions stability additive in the amount of 1xc3x9710xe2x88x924 M/l to 1xc3x9710xe2x88x922 M/l, (preferably 1xc3x9710xe2x88x923 M/l to 8xc3x9710xe2x88x923 M/l), it has been found that precipitation of trivalent chromium over time is reduced as evidence by the following example.